Beam Blocker Research Articles

Femtosecond laser darkened copper surfaces: Characterization for possible application as laser beam blocker

This study focuses on the optical and mechanical properties of copper surfaces darkened by femtosecond laser irradiation, with regard to their applicability as laser beam blockers. Copper surfaces were processed with a Ti-Sapphire laser (1 kHz repetition rate, ∼100–600 mJ/cm2 fluence), while scanning parameters were optimized for low reflectivity and processing speed. A total reflectivity below 6 % was obtained for the 250–2500 nm wavelength range. SEM imaging and ion beam etching showed submicron period structures covered with high density nanoparticles, with a total depth of ∼3–4 μm. Numerical reflectivity simulations suggest that besides the structures, the formation of oxides also contributes to the observed darkening. Thermal and mechanical stability, chemical resistance and laser damage threshold were studied. Temperatures above 200 °C caused surface degradation. Scratch tests showed that the nanostructured layers could withstand ∼60 MPa contact pressure. The surfaces could resist 48 h treatment with various acids at 10−3 M concentration. The laser damage threshold for the applied femtosecond laser showed that the surface could withstand the unfocused beam of most high-frequency femtosecond lasers. The results indicate that laser darkened copper can be a promising candidate for use in laser beam blockers.

A practical and robust method for beam blocker-based cone beam CT scatter correction

Objective. In the traditional beam-blocker based cone beam CT (CBCT) scatter correction, the scatter measured in the region shaded by lead strips was multiplied by a correction factor to directly represent the scatter in the unblocked region. The correction factor optimization is a tedious process and lacks an objective stop criterion. To skip the optimization process, an indirect scatter estimation method was developed and validated in phantom imaging. Approach. A beam-blocker made of lead strips was mounted between the x-ray source and object for scatter estimation. The primary signal between lead strips in the blocked region was first calculated by subtracting the measured scatter, and then used to calculate the scatter signal in the unblocked region corresponding to the same attenuation path. The calculated scatter signal was smoothed via local filtration and used to correct the measured projection in the unblocked region. Finally, the CBCT was reconstructed via Feldkamp–Davis–Kress algorithm. A Catphan and a head phantom were used to verify the performance of the proposed method in both full- and half-blocker scenarios, and with and without a bow-tie filter. Main Results. For scans without the bow-tie filter, the CT number error was reduced to 3.97 2.27 and 5.51 3.90 HU in the full- and half-blocker scenarios, respectively, for the Catphan, and to 4.01 2.18 and 7.97 ± 4.05 HU for the head phantom. When the bow-tie filter was applied, the CT number error was reduced to 2.29 1.42 and 6.72 0.77 HU in the full- and half-blocker scenarios, respectively, for the Catphan, and 2.35 1.25 and 4.96 ± 1.89 HU for the head phantom. Significance. The proposed method effectively avoids the influence of the inserted beam blocker itself on the scatter intensity estimation, and proves a more practical and robust way for the beam-blocker based scatter correction in CBCT scanning.

Generation of a focused nonlinear difference frequency using high-amplitude time reversal focusing of airborne ultrasound

Time reversal (TR) focusing of high-amplitude airborne ultrasound can be used to create a difference frequency. Methods are employed to increase the amplitude of the focus. These methods include creating a miniature reverberation chamber, using multiple sources, and using the clipping processing method. The use of a beam blocker to make the sources more omnidirectional is also examined, and it is found that for most source/microphone orientations, the use of a beam blocker increases the amplitude of the focus. A high-amplitude focus of 134 dB peak re 20 μPa sound pressure level with a center frequency of about 38 kHz is generated using eight sources. Then, using four sources centered at 36.1 kHz and another four sources centered a 39.6 kHz, nonlinear difference frequency content centered at 3.5kHz is observed in the focus signal. The difference frequency amplitude grows quadratically with increasing primary frequency amplitude. When using beam blockers, the difference frequency content propagates away from the focal location with higher amplitude than when beam blockers are not used. This is likely due to the differences in the directionality of the converging waves during TR focusing.

High-amplitude time reversal focusing of airborne ultrasound to generate a focused nonlinear difference frequency.

Time reversal (TR) focusing of airborne ultrasound in a room is demonstrated. Various methods are employed to increase the amplitude of the focus. These methods include creating a small wooden box (or chamber) to act as a miniature reverberation chamber, using multiple sources, and using the clipping processing method. The use of a beam blocker to make the sources more omnidirectional is also examined, and it is found that for most source/microphone orientations, the use of a beam blocker increases the amplitude of the focus. A high-amplitude focus of 134 dB peak re 20 μPa sound pressure level with a center frequency of about 38 kHz is generated using TR. Using four sources centered at 36.1 kHz and another four sources centered at 39.6 kHz, nonlinear difference frequency content centered at 3.5 kHz is observed in the focus signal. The difference frequency amplitude grows quadratically with increasing primary frequency amplitude. When using beam blockers, the difference frequency content propagates away from the focal location with higher amplitude than when beam blockers are not used. This is likely due to the differences in the directionality of the converging waves during TR focusing.

An isotope harvesting beam blocker for the National Superconducting Cyclotron Laboratory

Harvesting isotopes from beam stops and other activated materials at accelerator facilities is a promising source of environmentally, scientifically and socially important radionuclides. At the Facility for Rare Isotope Beams (FRIB), a multitude of short- and long-lived radionuclides will be collected in a synergistic manner by dumping unused beams into a flowing-water beam stop. Ongoing exploratory research at the National Superconducting Cyclotron Laboratory (NSCL) with an analogous beam blocker aims towards obtaining the necessary radiochemical expertise for this endeavor.Herein we present a beam blocker and an isotope harvesting system which allows collection of a wide variety of aqueous and gaseous radionuclides. The water which flows through the beam blocker functions as an isotope production target and concurrently transports the newly formed radionuclides to collection sites. The system includes analytical instruments for online measurements of conductivity, dissolved oxygen, temperature, pressure and for detection of radiolytic products. To limit the levels of radiolytically produced hydrogen peroxide, a stainless-steel based degradation system was designed and implemented. The suitability of the constructed system for the anticipated radionuclide harvesting project was demonstrated by offline tests and under irradiation with 140 MeV/u 48Ca20＋ ions at the NSCL Coupled Cyclotron Facility.

Scatter correction for a clinical cone-beam CT system using an optimized stationary beam blocker in a single scan.

Scatter contamination in the cone-beam CT (CBCT) leads to CT number inaccuracy, spatial nonuniformity, and loss of image contrast. In our previous work, we proposed a single scan scatter correction approach using a stationary partial beam blocker. Although the previous method works effectively on a tabletop CBCT system, it fails to achieve high image quality on a clinical CBCT system mainly due to the wobble of the LINAC gantry during scan acquisition. Due to the mechanical deformation of CBCT gantry, the wobbling effect is observed in the clinical CBCT scan, and more missing data present using the previous blocker with the uniformly distributed lead strips. An optimal blocker distribution is proposed to minimize the missing data. In the objective function of the missing data, the motion of the beam blocker in each projection is estimated using the segmentation due to its high contrast in the blocked area. The scatter signals from the blocker are also estimated using an air scan with the inserted blocker. The final image is generated using the forward projection to compensate for the missing data. On the Catphan©504 phantom, our approach reduces the average CT number error from 86 Hounsfield unit (HU) to 9HU and improves the image contrast by a factor of 1.45 in the high-contrast rods. On a head patient, the CT number error is reduced from 97 HU to 6 HU in the soft-tissue region and the image spatial nonuniformity is decreased from 27% to 5%. The results suggest that the proposed method is promising for clinical applications.

Dynamic fluence field modulation for miscentered patients in computed tomography.

Traditional CT image acquisition uses bowtie filters to reduce dose, x-ray scatter, and detector dynamic range requirements. However, accurate patient centering within the bore of the CT scanner takes time and is often difficult to achieve precisely. Patient miscentering combined with a static bowtie filter can result in significant increases in dose, reconstruction noise, and CT number variations, and consequently raise overall exposure requirements. Approaches to estimate the patient position from scout scans and perform dynamic spatial beam filtration during acquisition are developed and applied in physical experiments on a CT test bench using different beam filtration strategies. While various dynamic beam modulation strategies have been developed, we focus on two approaches: (1)a simple approach using attenuation-based beam modulation using a translating bowtie filter and (2)dynamic beam modulation using multiple aperture devices (MADs)-an emerging beam filtration strategy based on binary filtration of the x-ray beam using variable width slits in a high-density beam blocker. Improved dose utilization and more consistent image performance with respect to an unmodulated baseline (static filter) are demonstrated for miscentered objects and dynamic beam filtration in physical experiments. For a homogeneous object miscentered by 4cm, the dynamic filter reduced the maximum regional noise and dose penalties (compared with a centered object) from 173% to 16% and 42% to 14%, respectively, for a traditional bowtie, 29% to 8% and 24% to 15%, respectively, for a single MAD, and 275% to 11% and 56% to 18%, respectively, for a dual-MAD filter. The proposed methodology has the potential to relax patient centering requirements within the scanner, reduce setup time, and facilitate additional CT dose reduction.

Binary moving-blocker-based scatter correction in cone-beam computed tomography with width-truncated projections: proof of concept

This paper proposes a binary moving-blocker (BMB)-based technique for scatter correction in cone-beam computed tomography (CBCT). In concept, a beam blocker consisting of lead strips, mounted in front of the x-ray tube, moves rapidly in and out of the beam during a single gantry rotation. The projections are acquired in alternating phases of blocked and unblocked cone beams, where the blocked phase results in a stripe pattern in the width direction. To derive the scatter map from the blocked projections, 1D B-Spline interpolation/extrapolation is applied by using the detected information in the shaded regions. The scatter map of the unblocked projections is corrected by averaging two scatter maps that correspond to their adjacent blocked projections. The scatter-corrected projections are obtained by subtracting the corresponding scatter maps from the projection data and are utilized to generate the CBCT image by a compressed-sensing (CS)-based iterative reconstruction algorithm. Catphan504 and pelvis phantoms were used to evaluate the method’s performance. The proposed BMB-based technique provided an effective method to enhance the image quality by suppressing scatter-induced artifacts, such as ring artifacts around the bowtie area. Compared to CBCT without a blocker, the spatial nonuniformity was reduced from 9.1% to 3.1%. The root-mean-square error of the CT numbers in the regions of interest (ROIs) was reduced from 30.2 HU to 3.8 HU. In addition to high resolution, comparable to that of the benchmark image, the CS-based reconstruction also led to a better contrast-to-noise ratio in seven ROIs. The proposed technique enables complete scatter-corrected CBCT imaging with width-truncated projections and allows reducing the acquisition time to approximately half. This work may have significant implications for image-guided or adaptive radiation therapy, where CBCT is often used.

SU‐F‐J‐211: Scatter Correction for Clinical Cone‐Beam CT System Using An Optimized Stationary Beam Blocker with a Single Scan

Purpose:X‐ray scatter photons result in significant image quality degradation of cone‐beam CT (CBCT). Measurement based algorithms using beam blocker directly acquire the scatter samples and achieve significant improvement on the quality of CBCT image. Within existing algorithms, single‐scan and stationary beam blocker proposed previously is promising due to its simplicity and practicability. Although demonstrated effectively on tabletop system, the blocker fails to estimate the scatter distribution on clinical CBCT system mainly due to the gantry wobble. In addition, the uniform distributed blocker strips in our previous design results in primary data loss in the CBCT system and leads to the image artifacts due to data insufficiency.Methods:We investigate the motion behavior of the beam blocker in each projection and design an optimized non‐uniform blocker strip distribution which accounts for the data insufficiency issue. An accurate scatter estimation is then achieved from the wobble modeling. Blocker wobble curve is estimated using threshold‐based segmentation algorithms in each projection. In the blocker design optimization, the quality of final image is quantified using the number of the primary data loss voxels and the mesh adaptive direct search algorithm is applied to minimize the objective function. Scatter‐corrected CT images are obtained using the optimized blocker.Results:The proposed method is evaluated using Catphan@504 phantom and a head patient. On the Catphan©504, our approach reduces the average CT number error from 115 Hounsfield unit (HU) to 11 HU in the selected regions of interest, and improves the image contrast by a factor of 1.45 in the high‐contrast regions. On the head patient, the CT number error is reduced from 97 HU to 6 HU in the soft tissue region and image spatial non‐uniformity is decreased from 27% to 5% after correction.Conclusion:The proposed optimized blocker design is practical and attractive for CBCT guided radiation therapy.This work is supported by grants from Guangdong Innovative Research Team Program of China (Grant No. 2011S013), National 863 Programs of China (Grant Nos. 2012AA02A604 and 2015AA043203), the National High‐tech R&D Program for Young Scientists by the Ministry of Science and Technology of China (Grant No. 2015AA020917)

SU‐D‐206‐07: CBCT Scatter Correction Based On Rotating Collimator

Purpose:Scatter correction in cone‐beam computed tomography (CBCT) has obvious effect on the removal of image noise, the cup artifact and the increase of image contrast. Several methods using a beam blocker for the estimation and subtraction of scatter have been proposed. However, the inconvenience of mechanics and propensity to residual artifacts limited the further evolution of basic and clinical research. Here, we propose a rotating collimator‐based approach, in conjunction with reconstruction based on a discrete Radon transform and Tchebichef moments algorithm, to correct scatter‐induced artifacts.Methods:A rotating‐collimator, comprising round tungsten alloy strips, was mounted on a linear actuator. The rotating‐collimator is divided into 6 portions equally. The round strips space is evenly spaced on each portion but staggered between different portions. A step motor connected to the rotating collimator drove the blocker to around x‐ray source during the CBCT acquisition. The CBCT reconstruction based on a discrete Radon transform and Tchebichef moments algorithm is performed. Experimental studies using water phantom and Catphan504 were carried out to evaluate the performance of the proposed scheme.Results:The proposed algorithm was tested on both the Monte Carlo simulation and actual experiments with the Catphan504 phantom. From the simulation result, the mean square error of the reconstruction error decreases from 16% to 1.18%, the cupping (τcup) from 14.005% to 0.66%, and the peak signal‐to‐noise ratio increase from 16.9594 to 31.45. From the actual experiments, the induced visual artifacts are significantly reduced.Conclusion:We conducted an experiment on CBCT imaging system with a rotating collimator to develop and optimize x‐ray scatter control and reduction technique. The proposed method is attractive in applications where a high CBCT image quality is critical, for example, dose calculation in adaptive radiation therapy.We want to thank Dr. Lei Xing and Dr. Yong Yang in the Stanford University School of Medicine for this work. This work was jointly supported by NSFC (61471226), Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201516), and China Postdoctoral Science Foundation (2015T80739, 2014M551949).

WE‐G‐207‐08: Imaging Dose Reduction and Scatter Removal in Cone Beam CT Via Random Undersampling: A Simulation Study

Purpose:Cone‐beam CT (CBCT) is widely used in image‐guided radiation therapy. The high imaging dose from repeated uses is a clinical concern and its image quality is impeded by a large amount of scattered photons. We propose to solve these two problems via a random undersampling method. We have performed Monte Carlo (MC) simulation studies for an initial test of the method.Methods:We propose to place a moving beam blocker with a random blocking pattern in front of the x‐ray source. It blocks a projection with a random pattern, which varies among projections. Scatter signal is measured in the deliberately created shadows, which is further interpolated to the entire projection. After removing the interpolated scatter from the total signal in the un‐blocked area, the cleaned data were used for CBCT reconstruction under a Tight Frame (TF) based iterative method. The random sampling yields a projection matrix that has a better numerical property compared to regular undersampling, permitting better image reconstruction.Results:360 CBCT projections of a head‐and‐neck cancer patient were generated via MC simulations. We first reconstructed CBCTs using 90 full projections with only primary data. The RMS error was 16%. When scattered photons were included in the projection, reduced contrast was observed and the RMS error was 22%. In our proposed method, 360 projections were used, but each of them was randomly blocked by 75% of pixels. Scatter estimation and was performed and the corrected data were used in reconstruction, yielding a reconstruction error of 15%. In these two approaches, the imaging doses were both reduced by ∼75% compared to a full scan with 360 unblocked projections.Conclusion:we proposed a method to solve the imaging dose and scatter problem in CBCT in a unified manner. Preliminary simulation studies demonstrated its efficacy.

Direct IR excitation in a fast ion beam: application to NO- photodetachment cross sections

BackgroundOptical access to a travelling ion packet is required in many ion beam experiments that study ion-photon interactions.MethodsAn approach is described for carrying out direct infrared excitation of a fast ion beam that uses an optical-quality reflective beam blocker to illuminate a counter propagating pulsed ion beam in a collinear configuration. This arrangement provides optical access along the axis of ion beam propagation by placing a mirror in the beam path at a 25 degree angle. The ion packet is bumped over the mirror, which is also used to block fast neutral particles produced during ion beam acceleration that also propagate along the beam path.ResultsThe efficiency of this setup is demonstrated in a photodetachment experiment on NO- anions, where a photoinduced depletion of up to 90% of the beam is achieved in a single laser shot. To demonstrate the application of this configuration, the relative photodetachment cross section for NO- has been measured in the range of 2800 – 7200 cm-1. The measured relative cross section shows a set of sharp peaks that are identified as vibrational autodetachment resonances.ConclusionThe new setup paves the way for future experiments where parent anionic species are vibrationally excited via direct infrared excitation first and undergo photodetachment/photodissociation in a subsequent step.PACS33.15.Ry; 33.80.Eh; 33.80.-b

Low-Dose and Scatter-Free Cone-Beam CT Imaging Using a Stationary Beam Blocker in a Single Scan: Phantom Studies

Excessive imaging dose from repeated scans and poor image quality mainly due to scatter contamination are the two bottlenecks of cone-beam CT (CBCT) imaging. Compressed sensing (CS) reconstruction algorithms show promises in recovering faithful signals from low-dose projection data but do not serve well the needs of accurate CBCT imaging if effective scatter correction is not in place. Scatter can be accurately measured and removed using measurement-based methods. However, these approaches are considered unpractical in the conventional FDK reconstruction, due to the inevitable primary loss for scatter measurement. We combine measurement-based scatter correction and CS-based iterative reconstruction to generate scatter-free images from low-dose projections. We distribute blocked areas on the detector where primary signals are considered redundant in a full scan. Scatter distribution is estimated by interpolating/extrapolating measured scatter samples inside blocked areas. CS-based iterative reconstruction is finally carried out on the undersampled data to obtain scatter-free and low-dose CBCT images. With only 25% of conventional full-scan dose, our method reduces the average CT number error from 250 HU to 24 HU and increases the contrast by a factor of 2.1 on Catphan 600 phantom. On an anthropomorphic head phantom, the average CT number error is reduced from 224 HU to 10 HU in the central uniform area.

Shot planning and analysis tools on the NIF project

Shot planning and analysis tools (SPLAT) integrate components necessary to help achieve a high over-all operational efficiency of the National Ignition Facility (NIF) by combining near and long-term shot planning, final optics demand and supply loops, target diagnostics planning, and target fabrication requirements. Currently, the SPLAT project is comprised of two primary tool suites for shot planning and optics demand. The shot planning component provides a web-based interface to selecting and building a sequence of proposed shots for the NIF. These shot sequences, or “lanes” as they are referred to by shot planners, provide for planning both near-term shots in the Facility and long-term “campaigns” in the months and years to come. The shot planning capabilities integrate with the Campaign Management Tool (CMT) for experiment details and the NIF calendar for availability. Future enhancements will additionally integrate with target diagnostics planning and target fabrication requirements tools. The optics demand component is built upon predictive modeling of maintenance requirements on the final optics as a result of the proposed shots assembled during shot planning. The predictive models integrate energetics from a Laser Performance Operations Model (LPOM), the status of the deployed optics as provided by the online Final Optics Inspection system, and physics-based mathematical “rules” that predict optic flaw growth and new flaw initiations. These models are then run on an analytical cluster comprised of forty-eight Linux-based compute nodes. Results from the predictive models are used to produce decision-support reports in the areas of optics inspection planning, optics maintenance exchanges, and optics beam blocker placement advisories. Over time, the SPLAT project will evolve to provide a variety of decision-support and operation optimization tools.

TU-A-213CD-09: A Binary Moving Blocker-Based Scatter Correction Technique for Cone-Beam CT with Width-Truncated Projections

Purpose: A binary moving blocker‐based technique for scatter‐corrected CBCTreconstruction enabling a single scanning configuration is proposed, in which width‐truncated projections are acquired from a shifted detector. Methods: A beam blocker consisting of lead strips is mounted in front of the kV x‐ray tube and alternately moves in and out during the gantry rotation. The projections are acquired in two phases. Projections belonging to the first phase are attenuated by the blocker resulting in a stripe pattern from blocked and unblocked regions. The second phase includes unblocked region only allowing for a complete volumetric reconstruction. To derive scatter map from the blocked projections, an 1D B‐Spline interpolation/extrapolation is applied by using the detected information in shaded regions. The scatter map of even indexed projections is calculated by averaging two scatter maps that correspond to their adjacent odd indexed projections. Scatter‐corrected projections are obtained by subtracting the corresponding scatter maps from projection data and are utilized to generate a CBCTimaging by a compressed sensing (CS) based iterative reconstruction algorithm. The Catphan504 and pelvis phantoms were used to evaluate the performance of the proposed method. Results: The proposed binary moving blocker‐based technique markedly suppressed the scatter‐ and bone‐induced artifacts in CBCT. Compared with CBCT generated by non‐blocker based FDK algorithm, the non‐uniformity value was reduced from 39.3% to 2.1%. CT number errors in the regions of interest were reduced from 50 to less than 10. The CS based reconstruction algorithm provided the contrast‐to‐noise ratio with spatial resolution comparable to the benchmark image.Conclusions: The proposed technique enables complete volumetric scatter‐corrected CBCTimaging with width‐truncated projections and allows reducing the patient dose at partially‐shaded regions.

Scatter correction in cone‐beam CT via a half beam blocker technique allowing simultaneous acquisition of scatter and image information

X-ray scatter incurred to detectors degrades the quality of cone-beam computed tomography (CBCT) and represents a problem in volumetric image guided and adaptive radiation therapy. Several methods using a beam blocker for the estimation and subtraction of scatter have been proposed. However, due to missing information resulting from the obstruction of the blocker, such methods require dual scanning or dynamically moving blocker to obtain a complete volumetric image. Here, we propose a half beam blocker-based approach, in conjunction with a total variation (TV) regularized Feldkamp-Davis-Kress (FDK) algorithm, to correct scatter-induced artifacts by simultaneously acquiring image and scatter information from a single-rotation CBCT scan. A half beam blocker, comprising lead strips, is used to simultaneously acquire image data on one side of the projection data and scatter data on the other half side. One-dimensional cubic B-Spline interpolation/extrapolation is applied to derive patient specific scatter information by using the scatter distributions on strips. The estimated scatter is subtracted from the projection image acquired at the opposite view. With scatter-corrected projections where this subtraction is completed, the FDK algorithm based on a cosine weighting function is performed to reconstruct CBCT volume. To suppress the noise in the reconstructed CBCT images produced by geometric errors between two opposed projections and interpolated scatter information, total variation regularization is applied by a minimization using a steepest gradient descent optimization method. The experimental studies using Catphan504 and anthropomorphic phantoms were carried out to evaluate the performance of the proposed scheme. The scatter-induced shading artifacts were markedly suppressed in CBCT using the proposed scheme. Compared with CBCT without a blocker, the nonuniformity value was reduced from 39.3% to 3.1%. The root mean square error relative to values inside the regions of interest selected from a benchmark scatter free image was reduced from 50 to 11.3. The TV regularization also led to a better contrast-to-noise ratio. An asymmetric half beam blocker-based FDK acquisition and reconstruction technique has been established. The proposed scheme enables simultaneous detection of patient specific scatter and complete volumetric CBCT reconstruction without additional requirements such as prior images, dual scans, or moving strips.

Fully Automated Measurement of the Modulation Transfer Function of Charge-Coupled Devices above the Nyquist Frequency

The charge-coupled devices used in electron microscopy are coated with a scintillating crystal that gives rise to a severe modulation transfer function (MTF). Exact knowledge of the MTF is imperative for a good correspondence between image simulation and experiment. We present a practical method to measure the MTF above the Nyquist frequency from the beam blocker's shadow image. The image processing has been fully automated and the program is made public. The method is successfully tested on three cameras with various beam blocker shapes.

Ultra High Resolution Linear Ion Trap Orbitrap Mass Spectrometer (Orbitrap Elite) Facilitates Top Down LC MS/MS and Versatile Peptide Fragmentation Modes

Although only a few years old, the combination of a linear ion trap with an Orbitrap analyzer has become one of the standard mass spectrometers to characterize proteins and proteomes. Here we describe a novel version of this instrument family, the Orbitrap Elite, which is improved in three main areas. The ion transfer optics has an ion path that blocks the line of sight to achieve more robust operation. The tandem MS acquisition speed of the dual cell linear ion trap now exceeds 12 Hz. Most importantly, the resolving power of the Orbitrap analyzer has been increased twofold for the same transient length by employing a compact, high-field Orbitrap analyzer that almost doubles the observed frequencies. An enhanced Fourier Transform algorithm—incorporating phase information—further doubles the resolving power to 240,000 at m/z 400 for a 768 ms transient. For top-down experiments, we combine a survey scan with a selected ion monitoring scan of the charge state of the protein to be fragmented and with several HCD microscans. Despite the 120,000 resolving power for SIM and HCD scans, the total cycle time is within several seconds and therefore suitable for liquid chromatography tandem MS. For bottom-up proteomics, we combined survey scans at 240,000 resolving power with data-dependent collision-induced dissociation of the 20 most abundant precursors in a total cycle time of 2.5 s—increasing protein identifications in complex mixtures by about 30%. The speed of the Orbitrap Elite furthermore allows scan modes in which complementary dissociation mechanisms are routinely obtained of all fragmented peptides.

Scatter correction for full‐fan volumetric CT using a stationary beam blocker in a single full scan

Applications of volumetric CT (VCT) are hampered by shading and streaking artifacts in the reconstructed images. These artifacts are mainly due to strong x-ray scatter signals accompanied with the large illumination area within one projection, which lead to CT number inaccuracy, image contrast loss and spatial nonuniformity. Although different scatter correction algorithms have been proposed in literature, a standard solution still remains unclear. Measurement-based methods use a beam blocker to acquire scatter samples. These techniques have unrivaled advantages over other existing algorithms in that they are simple and efficient, and achieve high scatter estimation accuracy without prior knowledge of the imaged object. Nevertheless, primary signal loss is inevitable in the scatter measurement, and multiple scans or moving the beam blocker during data acquisition are typically employed to compensate for the missing primary data. In this paper, we propose a new measurement-based scatter correction algorithm without primary compensation for full-fan VCT. An accurate reconstruction is obtained with one single-scan and a stationary x-ray beam blocker, two seemingly incompatible features which enable simple and efficient scatter correction without increase of scan time or patient dose. Based on the CT reconstruction theory, we distribute the blocked data over the projection area where primary signals are considered approximately redundant in a full scan, such that the CT image quality is not degraded even with primary loss. Scatter is then accurately estimated by interpolation and scatter-corrected CT images are obtained using an FDK-based reconstruction algorithm. The proposed method is evaluated using two phantom studies on a tabletop CBCT system. On the Catphan©600 phantom, our approach reduces the reconstruction error from 207 Hounsfield unit (HU) to 9 HU in the selected region of interest, and improves the image contrast by a factor of 2.0 in the high-contrast regions. On an anthropomorphic head phantom, the reconstruction error is reduced from 97 HU to 6 HU in the soft tissue region and image spatial nonuniformity decreases from 27% to 5% after correction. Our method inherits the main advantages of measurement-based methods while avoiding their shortcomings. It has the potential to become a practical scatter correction solution widely implementable on different VCT systems.

Scatter-free CBCT Imaging using a Beam Blocker and an Incoherence-enhancing Compressed Sensing Method

Scatter-free CBCT Imaging using a Beam Blocker and an Incoherence-enhancing Compressed Sensing Method